The field of the invention is that of the long-term navigation of a carrier using an inertial navigation unit.
Various types of carrier such as aircraft, ships or land vehicles are known, whose navigation entails knowing the position, the speed and the attitude (heading, roll and pitch).
A modern inertial navigation unit generally comprises inertial sensors such as gyrometers and accelerometers which are fixed securely to the carrier (a unit known as a strapdown unit).
The gyrometers measure angular rotations and allow the control of an analytical platform that has to remain fixed in a geographic frame of reference. The accelerometers measure accelerations, which measurements are projected onto this analytical platform and then integrated a first time, to give the speed, then a second time to give the position. The attitude is obtained by extracting the three angles that convert from the frame of reference of the carrier to the frame of reference of the analytical platform.
The precision of an inertial navigation unit depends directly on the errors of the inertial sensors (gyrometers and accelerometers) and more specifically on the projection of these errors into the local geographic frame of reference.
For a strapdown navigation unit, the projection of these errors depends essentially on the path of the carrier, and can therefore not be controlled.
The precision of a strapdown navigation unit is therefore limited by the intrinsic precision of these sensors. In the case of long-term inertial navigation, the position errors depend in particular on the precision of the gyrometers.
In order to improve the long-term navigation precision of a strapdown inertial unit, the subject of the invention is a method of navigation by means of a strapdown inertial master box mounted on a mechanical device that allows this master box to be placed in various successive positions so as to average out the gyrometer errors projected into the local geographic frame of reference.
The attitude information output from the strapdown inertial unit allows the mechanical device to be controlled directly so as to successively place the master box in various positions which are practically fixed with respect to the local geographic frame of reference.
More specifically, the subject of the invention is a long-term navigation method using an inertial master box with which there is associated a tied frame of reference of axes Xm, Ym, Zm, mounted on a carrier for measuring its movements with respect to a geographical frame of reference of fixed directions along three axes Xg, Yg, Zg, characterized in that it comprises:
measurement actions consisting in constantly measuring, by means of the inertial master box, an orientation of the tied frame of reference in the geographical frame of reference;
positioning actions which consist in applying a sequence of cycles of eight turnings-over of the inertial master box each of which keeps the axis Ym in a direction parallel to the axis Yg, a succession of two turnings-over about the axis Xm being preceded and followed by a turning-over about the axis Zm, a succession of two turnings-over about the axis Zm being preceded and followed by a turning-over about the axis Xm.
The positioning actions make it possible to compensate for the measurement errors by inverting the sign along the axis Ym on each turning-over, by inverting the sign along the axis Xm on each turning-over about the axis Zm and by inverting the sign along the axis Zm on each turning-over about the axis Xm.
The measurement actions constantly taken then make it possible to compensate mutually for the errors on each of the axes Xm, Ym, Zm of the tied frame of reference so as to reduce the projections of errors onto the axes Xg, Yg, Zg by averaging out their contributions over one cycle.
A further subject of the invention is a long-term navigation device using an inertial master box mounted on a carrier to measure its movements with respect to a frame of reference of fixed directions along three axes Xg, Yg, Zg, characterized in that it comprises a first motor for causing a gimbal to pivot on a first axle secured to the carrier, a second motor for causing the inertial master box to pivot on a second axle perpendicular to the first axle and secured to the gimbal, and a processor for controlling the first and second motors by running a program which applies a sequence of cycles of eight turnings-over of the inertial master box each of which keeps an axis Ym tied to the inertial master box in a direction parallel to the axis Yg, a succession of two turnings-over about the first axle being preceded and followed by a turning-over about the second axle, a succession of two turnings-over about the second axle being preceded and followed by a turning-over about the first axle.
The first motor allows the inertial master box to be kept in a plane containing the axis Yg so as to make any position of the inertial master box insensitive to the changes in heading of the carrier. The second motor allows the axis Ym to be kept horizontal.